Structural Characterization of Exon 6 of the Rat IGF-I Gene

In rat liver, insulin-like growth factor I (IGF-I) mRNAs exist as two major size classes of 7.5–7.0 kb and 1.2–0.9 kb. The 7.5- to 7.0-kb IGF-I mRNAs predominate in some nonhepatic tissues of the rat. Because the previously reported sequences of rat IGF-I cDNAs and genomic clones account for only 2.1 kb of sequence, the majority of the sequence of 7.5- to 7.0-kb rat IGF-I mRNAs was unknown. Using a combination of nucleotide sequencing of genomic DNA and cDNA clones and Northern hybridization and RNase protection, we have characterized a 6,354-base-long 3′ exon (exon 6) of the rat IGF-I gene. The sequence of exon 6 establishes the previously unknown sequence of the 3′ end of the 7.5- to 7.0-kb rat IGF-I mRNAs, comprised predominantly of an unusually long 3′ untranslated sequence (3′UT). The long 3′UT contains multiple ATTTA, A(T)_nA, and (T)_nA sequences, as well as inverted repeats. These sequences may contribute to the shorter half-life of the 7.5- to 7.0-kb rat IGF-I mRNAs relative to the 1.2- to 0.9-kb forms that have been demonstrated previously in vitro and in vivo. We also demonstrate that the 7.5- to 7.0-kb rat IGF-I mRNAs are localized to the cytoplasm of rat liver, providing indirect evidence that they are mature and functional mRNAs.